A differential microphone that can receive a sound from outside and reduce noise included in the sound has been known. A mobile phone utilizing such differential microphone can obtain a sound signal having little noise, that is, such a sound signal that a person at the other end can readily listen to sounds produced by a speaker.
In order to cancel out vibration of noise transmitted to a diaphragm or to cancel out a signal of noise output from the diaphragm, the differential microphone has at least two openings through which sounds are input. As will be described in the following, techniques for efficiently reducing noise have been proposed for the differential microphone.
Japanese Patent Laying-Open No. 2007-195140 (Patent Document 1), for example, discloses a unit structure of a microphone that prevents foreign substances from entering the microphone. According to Japanese Patent Laying-Open No. 2007-195140 (Patent Document 1), the microphone includes a substrate having a circuit board, a sound-processing unit connected to the circuit board, an upper lid connected to the substrate, and a sound hole provided in a lateral side of the upper lid.
In addition, Japanese Patent Laying-Open No. 2001-268695 (Patent Document 2) discloses an electret capacitor microphone. According to Japanese Patent Laying-Open No. 2001-268695 (Patent Document 2), the electret capacitor microphone includes a ceramic package which holds a back electrode having an electret dielectric film stuck on its top surface or a diaphragm ring made of a metal material having a diaphragm film stuck, by mounting it on an upper-end surface. A metal material film constituting an input terminal surface is formed on an upper-end surface of a peripheral side wall of the ceramic package and an input conduction film is formed by extending the input conduction film from the input terminal surface to an internal surface of the peripheral side wall and a top surface of a bottom part. An IC bare chip including an impedance converting circuit is fitted to the bottom part of the ceramic package and the input conduction film is electrically connected to an input end of the IC bare chip. The electret capacitor microphone includes a capsule made of a metallic cylinder. The ceramic package is put in the capsule.
In addition, Japanese Patent Laying-Open No. 2007-201976 (Patent Document 3) discloses a directional acoustic device. According to Japanese Patent Laying-Open No. 2007-201976 (Patent Document 3), a microphone includes a housing in a hollow box shape, a diaphragm housed within the housing, and a plurality of sound paths connecting a space in front of the diaphragm within the housing to the outside of the housing. In such a microphone, porous materials are disposed in the respective sound paths so as to make acoustic resistances of the respective sound paths different from one another, so that acoustics passing through the respective sound paths reach the diaphragm simultaneously when the acoustics are simultaneously made incident from outside the housing to all of the sound paths.
In addition, Japanese National Patent Publication No. 07-95777 (Patent Document 4) discloses a two-way sound communication headphone. According to Japanese National Patent Publication No. 07-95777 (Patent Document 4), the headphone includes a housing, means connected to the housing and including a microphone for converting wearer's conversation to an electric signal, means connected to the housing and including a receiver for converting the received electric signal to a sound, and means including an earpiece assembly supported by the housing, for conveying the sound from the means for converting the received signal to a wearer's ear.
In addition, Japanese Patent Laying-Open No. 2007-60661 (Patent Document 5) discloses a silicon based capacitor microphone. According to Japanese Patent Laying-Open No. 2007-60661 (Patent Document 5), the silicon based capacitor microphone includes a metal case, and a substrate which is mounted with an MEMS (Micro Electro Mechanical System) microphone chip and an ASIC (Application Specific Integrated Circuit) chip having a voltage pump and a buffer IC and is formed with a connecting pattern, on its surface, for bonding with the metal case, the connecting pattern being bonded to the metal case.